refactor: Use torch APIs for ravel, tile, and outer tensorlib methods

[matthewfeickert]

Description

Resolves #1349

Adopt the torch API for ravel, tile, and outer for easier comparisons across backends as the PyTorch v1.8.0 release notes comment

New functions (most of them to improve numpy compatibility)

Note that in the case of outer the v1.8.0 docs note that for torch.ger (the current function)

This function is deprecated and will be removed in a future PyTorch release. Use torch.outer() instead.

As a result also bump the minimum compatible release number of PyTorch to 1.8 so that these APIs exist.

Checklist Before Requesting Reviewer

• Tests are passing (CI is failing for known reasons)
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use torch.ravel, torch.tile, torch.outer for PyTorch backend tensorlib methods
   - https://pytorch.org/docs/1.8.0/generated/torch.ravel.html
   - https://pytorch.org/docs/1.8.0/generated/torch.tile.html
   - https://pytorch.org/docs/1.8.0/generated/torch.outer.html
* Set minimum compatible PyTorch release to v1.8 to ensure APIs

[codecov]

Codecov Report

Merging #1354 (0ea12a2) into master (04b6c86) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1354   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files          63       63           
  Lines        3768     3768           
  Branches      538      538           
=======================================
  Hits         3674     3674           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.50% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/tensor/pytorch_backend.py	98.27% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 04b6c86...0ea12a2. Read the comment docs.

[matthewfeickert]

@lukasheinrich @kratsg let me know if you want this to go into a later release and not touch this now as it bumps up the minimum required release torch to v1.8.0 — the latest. I think this would still be okay to have in a patch release, but let me know your thoughts.

[kratsg]

just a "lol" and "lgtm" here